Added filtering by keywords to nf-core modules list

[ErikDanielsson]

Added new positional argument <keywords> to nf-core modules list which takes zero or more keywords and uses them to filter the retrieved modules. Should close #1139.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1152 (91aeaf1) into dev (6d30c3a) will decrease coverage by 0.01%.
The diff coverage is 63.15%.

@@            Coverage Diff             @@
##              dev    #1152      +/-   ##
==========================================
- Coverage   69.29%   69.27%   -0.02%     
==========================================
  Files          50       50              
  Lines        5224     5237      +13     
==========================================
+ Hits         3620     3628       +8     
- Misses       1604     1609       +5     

Impacted Files	Coverage Δ
nf_core/modules/list.py	66.21% <62.50%> (-0.46%)	⬇️
nf_core/__main__.py	56.91% <66.66%> (+0.11%)	⬆️
nf_core/modules/test_yml_builder.py	45.69% <0.00%> (-0.25%)	⬇️

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[KevinMenden]

LGTM 👍